Compact disks (CD), a type of optical disk, are widely used to store digital data of an audio signal or music information. The music is reproduced from the CD through a compact disk player. Optical disks have been adopted to store a variety of digital data besides audio signals. For instance, a CD-DA is used to store digital audio information; a CD-V is used to store video information such as movies, video clips and Keraoki. Another commonly recognized application of the optical disk in the data processing industry is the CD-ROM. The CD-ROM is a read-only data storage medium used in personal computers for storing digitized information such as large text databases and application programs. CD-ROMs are especially suitable for multimedia applications because of its voluminous storage capability, durability, and low cost. Although the traditional optical disk is a read-only medium used for data reproduction only, optical disks capable of both recording and reproduction have been developed. The CD-R and CD-E are both read/write optical storage mediums. An optical disk drive is used to record or reproduce the digital data onto or from these optical disks.
FIG. 1 shows a simplified block diagram of a typical prior art optical disk drive system used to reproduce data from a CD-ROM. In optical disk drive system 10, CD-ROM disk 12 is rotated by spindle motor 14; optical pickup unit 20 (denoted OPU in FIG. 1) reads the data stored on CD-ROM 12; feed motor (or "sled motor") 22 changes the radial position of optical pickup unit 20; microprocessor controller 40 and a collection of servo and control circuitry command disk drive 10 to perform the desired operations. CD-DSP 30 is a digital signal processor which descrambles the signal read from CD-ROM 12 by optical pickup unit 20 and provides, via CD-ROM controller 39, the digital output data via bus 44 to host computer 50. CD-ROM controller 39 is typically an ATAPI/IDE or SCSI based device, as is well known in the personal computer field.
A CD-ROM disk stores data in the form of pits and lands patterned in radial tracks. The tracks are formed in one spiral line extending from the inner radius of the disk to the outer edge. Unlike typical magnetic disk storage media which stores data in concentric tracks in which data density is more sparse at the outer edge of the disk, CD-ROM records data at a uniform density over its entire surface, that is the same high density of data is stored at an outer track as at an inner track. The uniform density storage pattern accounts for its large data storage capacity over that of the magnetic disk storage media. To facilitate data access in a uniform density storage pattern, the CD-ROM disk is designed to rotate at a constant linear velocity (CLV). In the CLV mode, the rotational speed of the disk varies according to the location of the track to be read so that a constant linear speed is maintained between the disk and the optical pickup unit. Maintaining a constant linear speed between the disk and the optical pickup at all locations on the disk means that the data transfer rate also remains constant at all locations, enabling a uniform density of data to be stored over the entire surface of the disk. In contrast, the magnetic disk storage media are designed to rotate at a constant angular velocity (CAV) where the rotational speed of the disk is kept constant. When operated in a CAV mode, the linear velocity of the disk relative to the optical pickup increases linearly as the read head approaches the outer track. The first CAV mode magnetic disk drives required that the amount of data recorded in a given angular rotation be kept the same throughout the disk. Therefore, the average data transfer rate will increase linearly as the optical pickup travels from the inner radius to the outer edge of the CD-ROM. More modern magnetic disk drives using CAV also use zone bit recording to improve storage capacity by increasing data density per angle of disk rotation as track location increases in distance from the center of the disk.
The advantage of high storage capacity of the CLV mode is offset by the longer access time required to reach a target track in a search operation as compared to the CAV mode. In the CLV mode, the time required to change the rotational speed of the disk whenever different tracks are accessed increases the access time significantly. When a CD-ROM is operated at 1.times. speed (representing the CD-ROM data transfer rate standard of 150 Kbytes/second), the rotational speed of the disk must change from approximately 500 rpm (rotations per minute) when an inner track is to be accessed to approximately 216 rpm when an outer track is to be accessed. In order to provide acceptable and jitter free performance, robust electronics are used in CD-ROM drives which allow reading data from a track when the spindle motor has reached its proper speed associated with that track, within a margin of .+-.50%. This allows reading of the desired track to occur somewhat sooner, and allows the final speed adjustments for that track to be made under closed loop control during the read operation. Since the data rate will not be correct when the motor speed is not correct, the electronics must be robust to allow for this improper data rate. To accommodate the frequent adjustments in rotational speed, a complex high performance, and thus expensive, spindle motor is required. The motor must be capable of generating more power, or producing more torque to reach a target speed quickly, and overcoming more friction. This is particularly true as CD-ROM operation speed increases. Today, it is common to run CD-ROMs at 8.times. speed or higher. At 8.times. speed, the rotational speed of the CD-ROM changes from near 4000 rpm at the inner track to about 1728 rpm at the outer track. At this rapid rotation rate, effective control of the speed of the motor becomes critical, especially for reducing the access time in a search operation. Furthermore, as CD-ROM speed increases, the design of the CD-DSP becomes increasingly difficult of the .+-.50% speed tolerance is to still be achieved. This .+-.50% guardband also limits the data rate possible under proper motor speed.
Conventional optical disk drive systems use open loop control of the motor speed in search operations and closed loop control during normal play or track following. In an open loop control, the spindle motor is driven in a kick and brake manner for changing the rotational speed as illustrated in FIG. 2. FIG. 2 shows a family of curves representing the rotational speed of the motor as a function of the radius of the optical disk. Curves 60, 62 and 64 depict the increase in rotational speed with respect to increasing CD-ROM speed from 8.times. to 16.times., when the CD-ROM is operated in a CLV mode across a variety of radiuses. When a track closer to the circumference of the disk is to be assessed, such as when the optical pickup must move from point A to point B, the motor must reduce the rotational speed of the disk. Therefore, a brake operation is initiated to decelerate the motor. Due to the presence of back EMF at the motor, it takes longer to accelerate the motor than to decelerate it a given change in rpm. Thus, the brake operation is typically terminated when the rotational speed is still above desired value, as indicated by point B1 in FIG. 2. Then, the rotational speed is further reduced to the desired value in fine increments by subsequent brake operations, until CD DSP 30 is able to synchronize with the sub-code bits stored on the desired track (typically when the speed reaches .+-.50% of the proper speed, due to the large tolerances built into the data recovery electronics), at which time closed loop speed control is used, based on the sub-code sync signal. It is highly undesirable to reduce the rotational speed too much as shown by point B2 because the motor must then be accelerated to the desired value at B by one or more kick operations, increasing the access time significantly. The converse is true when a track closer to the disk center is to be accessed, as illustrated by points C and D. The spindle motor must be accelerated as the optical pickup moves towards the center of the disk. The desired procedure is to accelerate the motor to above the desired value (i.e. to point D2) and then decelerate it in fine increments by subsequent brake operations to the desired speed (point D). This open loop control system is problematic because both time and power are wasted because the motor speed adjustments must be made with unavoidable overshoots and undershoots.
Contrary to the CLV mode, the CAV mode is characterized by very fast access time, i.e. the time required to move the optical pickup radially to a desired track and establish valid reading of data from that track at a tolerable spindle motor speed. Because the rotational speed is kept constant, the optical pickup can immediately retrieve disk data when the target track is reached. CAV mode operates without the added delay of waiting for the spindle motor speed to settle to the correct rpm.
In order to improve the data rate and access time of an optical disk drive under certain circumstances, various operating modes other than simply CLV have been suggested. FIG. 3-1 shows a graph depicting, in relative terms, the rotational speed and resulting data rate in a CLV mode of operation, as tracks are read from the inner diameter to the outer diameter of the CD-ROM. Since it is a CLV system, the rotational speed S1 decreases from the inner diameter to the outer diameter of the CD-ROM, and the data rate remains substantially constant. The substantially constant data rate makes for relatively simple data recovery circuitry, although as previously described the fact that a different rotational speed S1 is required for different tracks results in delays in access time as track changes are made.
Another prior art system, such as described in U.S. Pat. No. 5,388,085, uses a modified CAV (MCAV) method wherein the disk is rotated at a constant angular velocity while the data transfer rate is varied according to the radial position of the track accessed. FIG. 3-2 depicts this constant angular velocity as curve S2, and the resulting data rate D2 which increases proportionally from the inner diameter to the outer diameter of the CD-ROM.
A more commonly used approach involves a combined CAV/CLV scheme where the optical drive operates in a CAV mode at the inner tracks and switches to a CLV mode at the outer tracks. FIG. 3--3 is a graphical representation of the rotational speed profile, and the resulting data rate, of such a CAV/CLV system. As shown in FIG. 3--3, the constant angular velocity at the inner diameter tracks, as depicted by curve S3, is limited by the rotational speed of the drive motor and related mechanical components. At a certain point, the data rate is limited by the electronic components in the data recovery circuitry, resulting in a maximum data rate, as depicted by curve D3. In order to not increase the data rate further, at this point, the CD-ROM is operated in a CLV mode, with the rotational speed decreasing as tracks closer to the outer diameter of the CD-ROM are read. Although the CAV/CLV scheme has a lower data transfer rate at the inner tracks due to the maximum rotational speed of the motor, and related mechanical constraints due to vibration, etc., the resulting in overall system performance is acceptable and greater than the data transfer rate under CLV control over the entire CD-ROM since the need for motor speed adjustments within the CAV operating portion of the curves of FIG. 3--3 is precluded.
U.S. Pat. No. 5,521,895 describes a system in which normal data reading is performed under CLV control, and changes in rotational speed are achieved by increasing the speed to a fixed CAV speed and subsequently decelerating to the speed necessary to continue the CLV operation at the new track location.
To reap the maximum benefit of a CAV/CLV operation scheme, there is still a need to have accurate control of the angular velocity of the motor, since as depicted in FIG. 3--3, rotational speed adjustments are needed when changing tracks within the CLV portion of curve S3, or changing between the CAV and CLV portions of curve S3. Note that it is common to break the CLV portion of curve S3 into a plurality of zones (not shown) so that the CLV portion of the curve is not truly linear, but rather is approximated in a stair-step fashion. Nonetheless, there are a number of discreet motor speed values within the CLV portion of curve S3, necessitating changes in the motor speed when reading tracks within different zones.
Today's disk drive systems are required to provide for a +/-50% tolerance in order to meet the jitter-free control requirement. That is, the optical pickup must be able to read data even if the rotational speed is +/-50% of the target speed. The large tolerance level imposes design constraints on the disk drive electronics. It is desirable to reduce the tolerance level so as to remove unnecessary design constraints, particularly as the speed of the CD-ROMs increase to 8.times. and greater. In order to maintain proper control of the motor speed, there is a need to monitor the actual rotational speed of the motor. Current disk drive systems rely on reading a subcode sync signal from the disk which indicates the linear velocity of the track being read, and thus motor speed. This method depends on the optical pickup unit being able to accurately read the subcode sync signal from the CD-ROM and the CD-DSP (FIG. 1) being able to quickly descramble the signal. This sub-code is formed of 98 bits, with one frame of data stored on the CD-ROM containing one of the bits forming the sub-code. Thus, 98 frames of data must be read in order to obtain the 98 bits forming the sub-code. This method is time consuming and becomes increasingly inaccurate as CD-ROM speed increases. Thus, it is desirable to have some other method of monitoring the spindle motor speed.
Regardless of the method used in the prior art for selecting the desired rotational speed associated with each track on a CD-ROM being read, changes in rotational speed are performed in open loop fashion, using kick and brake operations. As previously described, in the more sophisticated prior art systems, care is taken not to undershoot a desired rotational speed, since acceleration is more time and power consuming than deceleration. Thus, by virtue of this open loop control, and the successive brakes required to fine tune the deceleration to achieve the desired rotational speed, changes in rotational speed are time consuming, thereby having a deleterious effect on the access time of CD-ROMs.